[Bug c++/104426] New: -fsanitize=undefined causes constexpr failures

["pdimov at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104426

            Bug ID: 104426
           Summary: -fsanitize=undefined causes constexpr failures
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pdimov at gmail dot com
  Target Milestone: ---

The following program

```
struct category
{
    constexpr bool failed() const noexcept
    {
        return true;
    }
};

inline constexpr category s_cat;

struct condition
{
    category const* cat_;

    constexpr bool failed() const noexcept
    {
        if( cat_ )
        {
            return cat_->failed();
        }
        else
        {
            return false;
        }
    }
};

int main()
{
    constexpr condition cond{ &s_cat };
    static_assert( cond.failed() );
}
```

compiles without -fsanitize=undefined (https://godbolt.org/z/Pn9M5ocfz), but
fails with it (https://godbolt.org/z/KKc8Tb9qe) with

```
<source>: In function 'int main()':
<source>:31:31: error: non-constant condition for static assertion
   31 |     static_assert( cond.failed() );
      |                    ~~~~~~~~~~~^~
<source>:31:31:   in 'constexpr' expansion of 'cond.condition::failed()'
<source>:17:13: error: '((& s_cat) != 0)' is not a constant expression
   17 |         if( cat_ )
      |             ^~~~
```

This happens under all GCC versions starting from 7.

(The above is an extract from the test suite for
boost::system::error_condition.)